fix: Set TEXTPLAN_SOURCE_DIR based on current dir in ploadloader test.

That variable was previously computed based on `CMAKE_SOURCE_DIR`, which
is the wrong value if `substrait-cpp` is used as a dependency from a
different top-level CMake project. The PR applies the pattern based on
`CMAKE_CURRENT_SOURCE_DIR` used in other tests.

export/planloader/tests/CMakeLists.txt

@@ -15,7 +15,7 @@ add_test_case(
  gtest
  gtest_main)
 
-set(TEXTPLAN_SOURCE_DIR "${CMAKE_SOURCE_DIR}/src/substrait/textplan")
+cmake_path(GET CMAKE_CURRENT_SOURCE_DIR PARENT_PATH TEXTPLAN_SOURCE_DIR)
 
 add_custom_command(
  TARGET planloader_test